On the grounds of Alnwick Castle in Northumberland, a most unorthodox English garden is taking shape—complete with towering jets of water and a bit of strychnine.

The first thing you notice as you enter Alnwick Garden is all the water. Walk through the new pavilion and visitors' center—an airy, glass-and-wood structure designed by Sir Michael Hopkins—and you'll see a swath of greensward and, beyond it, what looks like a moving hillside. This is the Grand Cascade: 21 weirs, down which tumble 8,700 gallons of water per minute. It's just the first of a series of water sculptures that punctuate the grand, theatrical space, one of the most ambitious horticultural projects in Europe since Versailles. On the grounds of the 12th-century Alnwick Castle in Northumberland, 35 miles from the Scottish border in northeast England, the garden is laid out over 40 acres; when it is finished, sometime in the next three years, it will have cost $73 million.

"Every other cascade in the world is switched off in the winter," says Jane Percy, the Duchess of Northumberland, who's showing me around her garden. "But this one won't be. When this freezes, we will have lights that go on behind the icy water, so you will have this sheet of ice and water moving down the hill."

A little farther on, we stop at Vortex, one of eight water sculptures designed by British artist William Pye. Imagine a giant stainless-steel shaving bowl. In the bottom are two vents that propel water into the dish in opposite directions. As the dish fills, it creates a vortex. The sky is reflected in the surface of the water, and a group of people stand around the dish, mesmerized. Next comes Torricelli, a 16-foot-high stainless-steel column that slowly fills with water until it overflows down the mirrorlike sides. Several children are getting soaked as they press their hands against the column. Suddenly, 90 vertical jets surrounding the base of the column shoot water into the air. The children squeal. "This is what some people don't like about this project," says Jane, referring to the garden's crowd-pleasing accessibility. "But it's what I love about it."

For the Brahmins of the British horticultural establishment, squealing children are only marginally less popular than slugs. And ever since this neophyte announced her plans to build her garden for Everyman, these latter-day Gertrude Jekylls have had their pruning shears out for her. One tabloid called her "the Imelda Marcos of gardening." Most of the criticism has been driven by those two powerful motors of British society: envy and snobbery. Not only is the duchess telegenic, but as the wife of the Duke of Northumberland, she is also extremely rich. Add to this the fact that the family she married into is one of the oldest, most storied noble houses of Britain—a famous family forebear, Sir Harry "Hotspur" Percy, was immortalized by Shakespeare in Henry IV—and it is easy to see why she has attracted so much flak. Lady Mary Keen, one of the doyennes of British gardening, called Alnwick a vulgar theme park.

As we head toward the Labyrinth, a maze of fargesia bamboo designed by Adrian Fisher, who created the Maize Maze in Virginia among several others, Jane responds: "I say every garden has a theme. Some, it's white and blue borders. Some, it's yellow and blue. The theme here is water. I see it as a pleasure garden, something that's affordable for everybody."

Most visitors to gardens expect flowers. At Alnwick, you don't quite have to go searching for blooms, but flowers are not what the garden is about. "There are no flowers down here on the lower level, except for the Rose Garden," Jane explains. "Everything is about bones and shape. It took our gardening team by surprise. They couldn't understand why there was so little color. In the end, they were planting geraniums in the flower beds. I said, 'That's like van Gogh sticking a rose in the middle of his sunflower pictures.' You don't do it. You've got to remain true to the design."

There have been gardens at Alnwick Castle since the 18th century. The first gardener to leave his mark here was a local, the great Lancelot "Capability" Brown, who landscaped the adjoining park. In the 19th century, Czar Alexander I of Russia so admired Alnwick's hothouses and conservatories during a visit that he poached the head gardener and took him back to St. Petersburg. But by the 1950's, the garden that had once supplied pineapples to the British ambassador in Paris had fallen into disrepair. The Ornamental Garden, a beautiful space enclosed in brick walls, was used as a nursery to supply trees for the duke's estates. Today, it is a riot of color with chest-high delphiniums, roses, and dozens of varieties of perennials.

Jane gleefully recalls the visit of her most vociferous critic, Lady Mary Keen. "I happened to have on a pair of really baggy Dolce & Gabbana brown cords, with zips everywhere," she says. "She said, 'You look like Posh Spice.' And she kept complaining. As we were going up the stairs by the Grand Cascade, she said, 'This is a design error; every time I go up a step, I am using the same foot.' 'Well—change feet!' I said."

It's this no-nonsense attitude that has won Her Grace both friends and enemies. She never expected to become the Duchess of Northumberland. The rules of primogeniture meant that her husband's elder brother, Harry, was destined to inherit the family title. But in 1995, her brother-in-law, who had a history of depression, died after an overdose of amphetamines. Jane was at the hairdresser's when the news came through, and she found herself catapulted into the limelight as the 12th Duchess of Northumberland. "I had a lovely house and four children," she recalls. "And suddenly we had to move to the castle. The children didn't want to move here at first, either. But my husband had been brought up here, and he felt we had to be at the center of the estate." The garden was her reward.

FOUR years after its official opening by Prince Charles in 2002, and despite its being still unfinished, Alnwick is attracting some 500,000 visitors per year, making it the third-most-visited garden in Britain, after London's Kew Gardens and the Royal Horticulture Society's Garden at Wisley. The positive effects for the region, one of the most economically depressed in the country, have been notable. Hotels are full. Local shops are booming. Dozens of farmers, whose livelihoods were decimated by foot-and-mouth disease, have opened bed-and-breakfasts.

Alnwick Garden is still very much a work in progress. A giant tree house—6,000 square feet of Canadian cedar, Scandinavian redwood, and English and Scotch pine held aloft by 17 lime trees, with a restaurant inside—had just opened when I visited. And Jane is already busy fund-raising for the next stage. Plans include a Garden of the Senses, a Spiral Garden, and an orchard with 350 tai haku cherry trees.

One of the most talked-about features of the existing setup is the Poison Garden, which opened in February 2005. "Every garden has an apothecary garden," Jane says as we walk back down the hill from the Ornamental Garden, then through wrought-iron gates decorated with skulls and crossbones. She wanted something different. Some of the plants here are so dangerous that they have to be grown in cages: ricin, used by the Aum Shinrikyo cult on the Tokyo subway in 1995, and henbane, the poison dropped into the ears of Hamlet's father. "Strychnine is an interesting one," Jane says, leading me over to the gruesomely named Strychnos nux-vomica. "Children don't care how a plant cures," she says with a laugh. "They think that's boring. They want to know how it kills—and how agonizing the death is!"